Downloaded from http://bjsm.bmj.com/ on August 10, 2017 - Published by group.bmj.com Review The foot core system: a new paradigm for understanding intrinsic foot muscle function 1 2 3 4 Editor’s choice Patrick O McKeon, Jay Hertel, Dennis Bramble, Irene Davis Scan to access more free content ▸ To read the full version of ABSTRACT and global movers such as latissimus dorsi.2 The local this paper, please visit the The foot is a complex structure with many articulations stabilisers have small cross-sectional areas and small journal online (http://dx.doi. org/10.1136/bjsports-2013- and multiple degrees of freedom that play an important moment arms. Therefore, they do not produce large 092690). role in static posture and dynamic activities. The rotational moments at the respective joints that they evolutionary development of the arch of the foot was cross. However, they do act to increase intersegmen- 1Department of Exercise and Sport Sciences, School of coincident with the greater demands placed on the foot tal stability. Proper function of local stabilisers pro- Health Sciences and Human as humans began to run. The movement and stability of vides a stable base on which the primary movers of Performance, Ithaca College, the arch is controlled by intrinsic and extrinsic muscles. the trunk, those with larger cross-sectional areas and Ithaca New York, USA However, the intrinsic muscles are largely ignored by moment arms, can act to cause gross motion. When 2Department of Kinesiology, Curry School of Education, clinicians and researchers. As such, these muscles are core muscles are weak or are not recruited appropri- University of Virginia, seldom addressed in rehabilitation programmes. ately, the proximal foundation becomes unstable and Charlottesville, Virginia, USA Interventions for foot-related problems are more often malaligned, and abnormal movement patterns of the 3 Department of Biology, directed at externally supporting the foot rather than trunk and lower extremity ensue.3 This can lead to a – University of Utah, Salt Lake training these muscles to function as they are designed. In variety of overuse lower extremity injuries.4 7 City, Utah, USA 4Department of Physical this paper, we propose a novel paradigm for We propose that the concept of core stability Medicine and Rehabilitation, understanding the function of the foot. We begin with an may also be extended to the arch of the foot. The Spaulding National Running overview of the evolution of the human foot with a focus arch is controlled with both local stabilisers and Center, Harvard Medical on the development of the arch. This is followed by a global movers of the foot, similar to the lumbopel- School, Cambridge, Massachusetts, USA description of the foot intrinsic muscles and their vic core. The local stabilisers are the four layers of relationship to the extrinsic muscles. We draw the plantar intrinsic muscles that originate and insert Correspondence to parallels between the small muscles of the trunk region on the foot. These muscles generally have small Dr Patrick O McKeon, that make up the lumbopelvic core and the intrinsic foot moment arms, small cross-sectional areas and serve Department of Exercise and muscles, introducing the concept of the foot core. We primarily to stabilise the arches. The global movers Sport Science, School of Health Sciences and Human then integrate the concept of the foot core into the are the muscles that originate in the lower leg, Performance, Ithaca College assessment and treatment of the foot. Finally, we call for cross the ankle and insert on the foot. These Hill Center, Room G66 953 an increased awareness of the importance of the foot muscles have larger cross-sectional areas, larger S. Danby Rd, Ithaca, NY core stability to normal foot and lower extremity function. moment arms, are prime movers of the foot, and 14850, USA; [email protected] also provide some stability to the arch. With each footstep, the four layers of intrinsic muscles act to Accepted 27 February 2014 The human foot is a very complex structure, which control the degree and velocity of arch deform- allows it to serve many diverse functions. During ation. When they are not functioning properly, the standing, it provides our base of support. During foundation becomes unstable and malaligned; and gait, the foot must be stable at foot-strike and abnormal movement of the foot ensues. This may push-off. However, during mid-support, the foot manifest in foot-related problems. Plantar fasciitis must become a mobile adaptor and attenuate loads. It is one of the most common overuse injuries of the also possesses spring-like characteristics, storing and foot. It is recognised as a repetitive strain injury releasing elastic energy with each foot-strike. This is from excessive deformation of the arch.8 The accomplished through the deformation of the arch, importance of the arch musculature in this preva- which is controlled by intrinsic and extrinsic foot lent foot injury is currently underappreciated. This muscles. There is evolutionary evidence that the foot is underscored by recent articles describing clinical arch architecture and musculature developed in evidence and guidelines for plantar fasciitis,9 as response to the increased demands of load carriage well as posterior tibial tendon dysfunction,10 and running. The stability of this arch, which we pro- medial tibial stress syndrome11 and chronic lower posed to be the central ‘core’ of the foot, is requisite leg pain12 that have no mention of foot strengthen- to normal foot function. ing as a component of the interventions. Therefore, our purpose was to propose a foot THE RELEVANCE OF CORE STABILITY core system paradigm by (1) describing the evolu- TO THE FOOT tion of the human arch for locomotion, (2) delin- Core stability has received much attention in the clin- eating the subsystems of the foot core, (3) ical and athletic arenas. Interest has primarily been reviewing assessment and treatment of the foot focused on the role of lumbopelvic-hip stability in integrating the concepts of foot core stability and 1 fi To cite: McKeon PO, normal lower extremity movement patterns. The (4) nally discussing future research directions. Our Hertel J, Bramble D, et al. muscular system of the lumbopelvic hip complex, or overall goal was to propose a new paradigm by Br J Sports Med core, has been described as consisting of local stabili- which to view foot function, assessment and 2015;49:290. sers such as the multifidus and transverse abdominis, treatment. McKeon PO, et al. Br J Sports Med 2015;49:290. doi:10.1136/bjsports-2013-092690 1of9 Downloaded from http://bjsm.bmj.com/ on August 10, 2017 - Published by group.bmj.com Review THE ORIGIN OF THE HUMAN ARCH neural subsystems controlling spinal motion and stability origin- The human foot has evolved from one similar to that of African ally proposed by Panjabi.25 The passive subsystem consists of the apes, where it serves in both arboreal and terrestrial locomotion.13 bony and articular structures, while the active subsystem consists The transition from ape-like to human-like foot structure reflects a of the muscles and tendons attaching to and acting on the spine. shift to locomotor habits dominated by terrestrial bipedality. The neural subsystem consists of sensory receptors in the joint When walking bipedally, the gait of chimps is compromised by the capsules, ligaments, muscles and tendons surrounding the spine. absence of structural specialisations that permit the human foot to The passive subsystem provides for a balance between mobility operate as a compact, rigid lever system during the latter half of and stability of the vertebral column. The active subsystem con- stance. These include (1) an enlarged and permanently adducted sists of two functional muscular components: the local stabilisers hallux, (2) shortened lateral digits, (3) compaction and realign- and the global movers.2 The local stabilisers consist of both the ment of the tarsal bones to help prevent the ‘mid-tarsal break’ short, intersegmental muscles that primarily originate and insert observed in the foot of apes14 and (4) the addition of a well- on the spinal column and have short moment arms and act to defined medial longitudinal arch defended by strong plantar increase intersegmental dynamic stability. Proper function of tensile elements. The condition of the foot arches, absent in apes, local stabilisers provides a stable base on which the primary remains controversial. A transverse arch was likely present,15 but movers of the trunk can act to cause gross motion. The global the crucial medial longitudinal arch was absent or weakly movers cross multiple vertebral segments, have attachments on expressed,16 implying a poorly defined plantar aponeurosis and the pelvis and thorax, and can exert longer moment arms to hence greater reliance on muscular effort to resist forces acting on move the trunk and extremities. These include the more superfi- the toes during late stance. In contrast, even the earliest members cial erector spinae as well as the internal and external oblique of the genus Homo for which there is adequate evidence (eg, early and rectus abdominus muscles. The neural subsystem monitors Homo erectus) possessed an essentially modern foot structure, spinal motion and forces and sends afferent signals to the central including a well-defined medial longitudinal arch.17 nervous system. If those afferent signals exceed a given threshold, The modern human body (ie, Homo), especially in musculoskel- efferent signals are sent from the central nervous system to the etal design, reflects the mechanical demands of endurance appropriate muscles to alter spinal motion and forces. running.18 That habitual bipedalism was practiced for several million Hodges26 further delineated the strategies of lumbopelvic years by Australopithecus without the hallmark features of the core stability into ‘control’ and ‘capacity’ components.